I have made similar points about the time periods of Daniel, as being motivated primarily by symbolic numbers (70 periods of 7 years, or 10 jubilees in Daniel 9, and the subdivision of the final 7 years into equal halves of 3 1/2 years), expressing a periodizied view of history rather than a strictly chronological one (tho of course the periods are anchored to certain events which may be dated).
From this point of view, Jeremiah was spot-on in his predictions about the political realities. Not only did he reject the hopes that the exiles would return home soon and that Judah would gain its independance from the Babylonian "yoke", but he predicted that the situation was to last into the next generation, but not beyond that. Of course, this prediction was based on a pre-existing 70-year motif but history proved him correct (better than some of our modern political prognosticators). In other respects, as PP points out, his expectations did not turn out the way he had hoped. This was part of the reason why the author of Daniel in ch. 9 sought to reinterpret the expectations surrounding Jeremiah's 70-year prophecy.
I do feel however that Jeremiah reckoned his period of Babylonian supremacy (29:10) and vassalge of surrounding nations (25:11) at some definite point, and the wording in ch. 25 (the earlier prophecy) strongly suggests that 605 BC (v. 1) is the terminus a quo for the 70 years, i.e. "for twenty-three years ... until today" (v. 3), and this was about the time Jehoiakim became a vassal (tho this is debatable). The letter to the exiles in ch. 29 however dates to after 597 BC (v. 1-2), some 8 years later and may reflect a later conceptualization of the 70 years, and the reference to the 70 years as a period reserved "for Babylon" has as its focus Babylon's status (compare Daniel 4:29, which states that God "confers kingship on whom he pleases," with the implication in 29:10 that 70 years are granted to Babylon), while the reference to the 70 years in ch. 25 has the status of Judah and "surrounding nations" as its focus. It is then an exegetical matter of whether Jeremiah in ch. 29 was talking of Babylon's status as a political power in general (suggested by the wording itself) or its status as ruler over Judah and other nations (suggested by the connection to ch. 25). The fact is that history is messy in the details and transitions of power often take time over a period of years (look at the situation in Iraq), and different nations came under Babylonian rule at different times; similarly, Jerusalem and Judah underwent successive desolations and devastations and deportations over time, so it becomes a somewhat arbitrary matter of deciding between 612 or 609 BC as the "end of Assyria", for this "end" was really a process spanning several years. 609 BC however is no less a candidate than 612 BC (in disagreement with PP), if one were to pick a date, because this year marked the final end of Assyria as an independent nation. I am reminded of the way the final week is reckoned in Daniel 9; the high priest Onias III was deposed in 174 BC but the critical event that signals the final period of the era is his assassination (being "cut off") several years later in 171 BC. Apparently for the author of Daniel, it was this event (the end of the legitimate priesthood in his death) that had greater significance than his mere removal from power. So it is an open question of when Babylon could have been conceived as emerging as world power....612 BC? (when Assyria lost the bulk of its political power, along with its capital) 609 BC? (when Assyria was finally fully vanquished) 605 BC? (when Babylon defeated Egypt to become the supreme power in the Near East), etc. Any number of these can be entertained because of the duration it took for Babylon to gain political power. If one wants to pick a date that represents the beginning of Babylon as an unchallenged power, 605 BC would be the best date.
Of course, none of this impinges on the factual basis of Neo-Babylonian chronology, which is an entirely separate subject (i.e. none of the varying ways to assume a terminus a quo for the 70 years assumes a different chronology than the one discussed here).